1. Field of the Invention
The subject invention relates to a circuit for the correction of horizontal distortion in a raster display of a large deflection angle cathode ray tube (CRT).
2. Description of Related Art
In television receivers, there exists a distortion in the horizontal direction known as side pincushion distortion. In smaller deflection angle CRTs, this distortion may be corrected in the deflection coils. However, for larger deflection angle CRTs, additional circuitry must be added to correct the side pincushion distortion.
The side pincushion distortion may be corrected by increasing the line deflection current at the center of the raster and/or decreasing the line deflection current at the top and bottom of the raster. A diode modulator circuit is generally used for this purpose and is described in detail in U.S. Pat. No. 4,182,978 to Boekhorst, which is hereby incorporated by reference. As shown in FIG. 2, a typical diode modulator includes a horizontal output transistor T1 which is driven by a driver circuit (not shown). The emitter of transistor T1 is grounded while the collector is connected to one end of a series arrangement of two diodes D1 and D2, one end of a series arrangement of capacitors C1 and C2 (the other ends of both series arrangements being connected to ground), and to the line deflection coil Ly. The other end of the coil Ly is connected to a capacitor C3 which, in turn, is connected to the junctions of diodes D1 and D2 and of capacitors C1 and C2, and also to a one end of a series arrangement of inductors L1 and L2, the junction thereof being connected to ground via a capacitor C4. The free end of inductor L2 is connected to ground at node B via a capacitor C5. A voltage impressed at node B modulates the line deflection current. A larger voltage at node B will cause the horizontal width to decrease more than a smaller voltage. Hence, in order to correct the raster distortion of FIG. 1, a voltage waveform having a parabolic shape and the correct polarity is impressed at node B, as shown in FIG. 2. This is available from the vertical deflection output circuit in a television receiver which is then processed by a diode modulator driver stage.
A typical diode modulator driver stage is shown in FIG. 3 and may be represented by an operational amplifier A1 operating in an input summing configuration. This particular configuration is used because, due to the nature of the output voltage required at node B, several inputs are needed. The vertical parabola is obtained from the combination of capacitor C6 and resistor R1. Due to the design of the vertical output deflection amplifier, the voltage signal at the top of capacitor C6, with respect to ground, has a vertical ramp superimposed with the parabola. By proper choice of resistor R3 and the position of the wiper on variable resistor R2, it is possible to remove the ramp component from the voltage signal at node B or to allow either a small amount of positive going or negative going ramp to appear at node B giving the voltage waveform at node B a distinctive inclination to correct for trapezoid distortion in the raster. In addition to the input parabola and ramp, there is summed into node D a DC voltage from resistors R4 and R5 which allows the DC value of the voltage waveform at node B to be varied, thus allowing the horizontal width of the raster to be varied. Dynamic width regulation is obtained by resistor R6 which senses the beam current of the ultor anode and compensates for the increase in horizontal width due to the falling ultor anode voltage with an increase in beam current.
While the circuits of FIGS. 2 and 3 adequately correct the side pincushion distortion shown in FIG. 1, applicant has noticed that in certain CRTs, the correction of the side pincushion distortion leads to a distortion of the raster in the corner regions, as shown in FIG. 4.